Buffers are formed by an acid and its conjugate base (in this case, it should be citric acid-sodium citrate, but it dependes on the pH you want to keep).

Nevertheless, if removing NaOH didn’t fix the issue, then the problem is different. Can you make a sample with only 0.5% STPP to see if the amount of precipitate is less? Also, usually you mix SLS with SLES at a ratio of 1:2 or 1:3 to reduce water hardness sensitivity of SLS.